This invention relates to relatively high density and high purity boron nitride-boron nitride (BN-BN) composites. More particularly, this invention relates to a composite body prepared from boron nitride fibers and partially nitrided precursor fibers.
Boron nitride possesses a number of highly desirable properties which render it useful in a wide variety of applications. Its high electrical resistivity coupled with its high thermal conductivity make it especially useful in electrical and electronic applications requiring a material which simultaneously acts as an electrical insulator and a thermal conductor. Its excellent thermal shock resistance renders it effective as a refractory at temperatures up to 1600.degree. C or higher in a non-oxidizing atmosphere, and at temperatures as high as 700.degree. to 900.degree. C in air. It is highly corrosion resistant, being inert to most organic liquids and many corrosive chemicals and displaying excellent resistance to attack by various molten metals. Furthermore, because of its low dissipation factor over a wide temperature range this material is well suited for use as microwave and radar dielectric components (radar windows).
In U.S. Pat. No. 3,429,722, assigned to the same assignee as the present invention, James Economy et al have disclosed boron nitride fibers and a method of making them which comprises heating boric oxide fibers in an ammonia atmosphere under specified conditions. In accordance with the teachings of this patent, fibers incorporating a boron and nitrogen containing containing composition may be produced by reacting, with a nitrogen and hydrogen containing composition, fibers of B.sub.2 O.sub.3 having a maximum diameter of 20 to 30 microns. Preferably, boric oxide fibers with a maximum diameter of about 10 microns are heated in an ammonia atmosphere according to a specified temperature program to produce fibers consisting essentially of boron nitride.
In U.S. Pat. No. 3,668,059, James Economy et al have disclosed a high modulus boron nitride fiber having a diameter of less than 10 microns, and a Young's modulus of elasticity of at least about 15 .times. 10.sup.6 psi., prepared by heating a partially nitrided fiber consisting essentially of boron, oxygen, hydrogen and nitrogen at a temperature of at least 1800.degree. C under longitudinal tension.
In copending application, U.S. Ser. No. 124,919, filed Mar. 16, 1971, now U.S. Pat. No. 3,837,997, issued Sept. 24, 1974, Economy et al disclose a low density product comprising boron nitride fibers bonded by boron nitride formed in situ. The porous fibrous product is formed by heating a mass of boron nitride fibers that has been impregnated with boric acid solution to an elevated temperature in ammonia.
In addition to the above-noted references relating to boron nitride fibers, shaped boron nitride bodies have also been prepared in the past. Such articles are disclosed, for example, by Taylor, U.S. Pat. No. 2,888,325, which teaches the use of a multiple stage nitriding process comprising intermittant addition of oxygen-containing boron compound at intermediate stages of nitriding, followed by further nitriding.
Boron nitride bodies are generally fabricated by press-molding and sintering techniques, broadly classifiable as hot-pressing, and cold-pressing followed by sintering. Boron nitride bodies produced by hot-pressing techniques generally have a high boron oxide (B.sub.2 O.sub.3) content. While this material acts as a binder during hot pressing, and allows such bodies to be produced, it also causes a weakening of the properties of the resultant bodies produced. Thus, because of the presence of this material, hot pressed boron nitride articles have been weak at high temperatures, exhibited a permanent expansion upon heating to 1800.degree. C and cooling to room temperature, and absorbed atmospheric moisture to the extent that the boric acid formed thereby can cause cracking upon exposure to rapid heating.
To overcome these problems, Mandorf et al, in U.S. Pat. No. 3,734,997, teaches treating hot-pressed boron nitride articles with a suitable solvent to lower boron oxide content, followed by sintering in an inert atmosphere at a temperature of from 1800.degree. C to 2100.degree. C.
Attempts at forming high-density boron nitride bodies have also included chemical vapor deposition techniques, whereby a boron nitride body is subjected to a gaseous nitriding atmosphere and heated to reaction temperatures.
These various techniques for densification pose a common problem: each requires penetration of the boron nitride body by a solvent or reactant material. As density increases, obviously the ability of the specified material to penetrate into the structure, and thus permit complete reaction, and/or removal of B.sub.2 O.sub.3, decreases. Accordingly, the preparation of high density boron nitride materials has been a time-consuming and expensive process.
Pyrolytic boron nitride having a density in excess of 1.90 g./cc. has been produced by the reaction of boron trichloride and nitrogen. However, this method has been unsuccessful in producing articles greater than 1/4 to 1/2 inch in thickness. Further, the properties of this material are extremely anisotropic in nature. For example, such material has a flexural strength of about 15,000 psi. at room temperature, perpendicular to the crystal plane orientation, but less than about 2500 psi. parallel to the crystal planes.